The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, New York, (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, New York (1988), both of which are hereby incorporated herein by reference.
Electronic packages extend from the integrated circuit chip, through the module, card, and board, to the gate and system. The integrated circuit "chip" is referred to as the "zero order package." This chip or zero order package enclosed within its module is referred to as the first level of packaging. The integrated circuit chip provides circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical integrity.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip, and consequently, multiple chips are required to perform the function. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, the second level package provides for thermal management, i.e., heat dissipation. Several cards may, in turn, be mounted on one board.
Cards and boards may be polymer based or ceramic based. A basic process for polymer based composite package fabrication is the "prepreg" process described by George P. Schmitt, Bernd K. Appelt and Jeffrey T. Gotro, "Polymers and Polymer Based Composites for Electronic Applications" in Seraphim, Lasky, and Li, Principles of Electronic Packaging, pages 334-371, previously incorporated herein by reference, and by Donald P. Seraphim, Donald E. Barr, William T. Chen, George P. Schmitt, and Rao R. Tummala, "Printed Circuit Board Packaging" in Tummala and Rymaszewski, Microelectronics Packaging Handbook, pages 853-922, also previously incorporated herein by reference.
In the "prepreg" process for polymeric electronic circuit package fabrication, a fibrous body, such as a non-woven mat or woven web, is impregnated with a laminating resin, i.e., an adhesive. This step includes coating the fibrous body with, for example, an epoxy resin solution, evaporating the solvents associated with the resin, and partially curing the resin. The partially cured resin is called a B-stage resin. The body of fibrous material and B stage resin is the prepreg. The prepreg, which is easily handled and stable, may be cut into sheets for subsequent processing.
Typical resins used to form the prepreg include epoxy resins, cyanate ester resins, polyimides, hydrocarbon based resins, and fluoropolymers. One type of prepreg is the FR-4 prepreg. FR-4 is a fire retardant epoxy-glass cloth material, where the epoxy resin is the diglycidyl ether of 2,2'-bis(4-hydroxyphenyl) propane. This epoxy resin is also known as the diglycidyl ether of bisphenol-A, (DGEBA). The fire retardancy of the FR-4 prepreg is obtained by including approximately 15-20 weight percent bromine in the resin. This is done by incorporating the appropriate amount of resins or other brominated compounds.
Still other bisphenol-A diglycidyl ether resins are used to form rigid circuit boards. Among the resins so used to produce "prepreg" for reinforced laminate compositions for circuit boards are the lower molecular weight bisphenol A diglycidyl ether epoxy resins, including bromine-substituted resins for imparting some degree of flame resistance as disclosed by U.S. Pat. No. 4,782,116. Such epoxy resins are of relatively low equivalent weight, in the area of 180 to 200, using non brominated resin for example, so that the epoxy group content is relatively high, i.e., each relatively short repeating unit contains two epoxy groups, which results in an increase in the dielectric constant of the compositions after curing.
Other epoxy resin formulations useful in providing prepregs include high functionality resins, such as epoxidized cresol novolacs, and epoxidized derivatives of tris (hydroxyphenyl) methane. The multifunctional epoxy resins are characterized by high glass transition temperatures, high thermal stability, and reduced moisture up take.
Phenolic cured epoxies, as Ciba-Giegy RD86-170.TM., Ciba-Giegy RD87-211.TM., Ciba-Giegy RD87-212.TM., Dow Quatrex.RTM. 5010.TM., Shell Epon.RTM. 1151.TM., and the like, are also used in forming pre-pregs. These epoxies are mixtures of epoxies, with each epoxy having a functionality of at least 2, a phenolic curing agent with a functionality of at least 2, and an imidazole catalyst.
Cyanate ester resins are also used in forming prepregs. One type of cyanate ester resin includes dicyanates reacted with methylene dianiline bis-maleimide. This product may also be reacted with compatible epoxides to yield a three component laminate material. One such laminate material is a 50:45:5 (parts by weight) of epoxy:cyanate:maleimide. Typical of cyanate ester resins useful in forming prepregs is the product of bisphenol-A dicyanate and epoxy, which polymerizes during lamination to form a crosslinked structure.
Another class of dielectric substrates are film-adhesive systems. These differ from the above described fiber-resin systems by the use of an adhesive bearing film. Both the adhesive and the film must be carefully selected and matched. This is because of the tendency of the two to separate. One polyimide used for the film to carry the adhesive in a film-adhesive system, is a polyimide based on diphenylene dianhydride, described in U.S. Pat. No. 4,725,484 to Kiyoshi Kumagawa, Kenji Kuniyasu, Toshiyuki Nishino, and Yuji Matsui for DIMENSIONALLY STABLE POLYIMIDE FILM AND PROCESS FOR PREPARATION THEREOF.
Some proposed adhesive mixtures contain substantial amounts of the epoxy resin relative to the dicyanate polymer(s), producing an even higher dielectric constant. Also in such compositions the glass transition temperature and processing or curing temperature generally are reduced to such an extent that the thermal stability of the cured prepregs or laminates is unsatisfactory for high temperature processing applications.
However, the presently known film-adhesive systems and fiber-adhesive systems suffer from shortcomings. For example, epoxy-glass systems have a relatively high dielectric constant, and relatively poor thermal stability, while polyimide-glass systems have a poor copper peel strength. Attempts to substitute polymeric fibers or films for the glass fibers have introduced problems of microcracking and poor mechanical properties.
Triazine polymers, both homo-polymers and copolymers have similar processability to FR-4 polyepoxide. Thus, potentially, triazine polymers could be used as a dielectric material in multi-layer electronic circuit packages in an analogous manner to FR-4 polyepoxide.
However, the use of triazine polymers as dielectrics has been limited because of their poor mechanical properties. Specifically, triazines are brittle. Triazines fracture and microcrack during such processing steps as drilling, punching, and thermal cycling.
A further drawback of polymerized dicyanates is that when used with particulate or fibrous fillers, for example, glass cloth or aramid fabrics, the dielectric constant is increased to the range of 3.2 to 3.5. This is unsatisfactory for high clock speed microelectronic applications.